FIG. 9(a) is a perspective view illustrating a prior art semiconductor light intensity modulator and FIG. 9(b) is a cross sectional view thereof taker along line D-D' of FIG. 9(a). In the figure, this prior art semiconductor light intensity modulator has an n type InP substrate 1 having an energy band gap corresponding to light of wavelength of .lambda.g=0.9 .mu.m. A light absorption layer comprising undoped InGaAsP having an energy band gap corresponding to light of wavelength .lambda.g=1.4 .mu.m is grown on the n type InP substrate 1. A p type InP layer 3 having an energy band gap corresponding to wavelength of .lambda.g=0.9 .mu.m is grown on the light absorption layer 2. A p side electrode 4 is produced on the p type InP layer 3. An n side electrode 5 is produced on the n type InP substrate 1. The n side electrode 5 is grounded to the earth 6. A modulation signal 7 is input to the p side electrode 4.
A method of producing this prior art semiconductor light intensity modulator will be described hereinafter.
First of all, an undoped InGaAsP layer of 0.13 .mu.m thickness having an energy band gap absorbing light of wavelength .lambda.g=1.4 .mu.m is epitaxially grown on the n type InP substrate 1 having a dopant concentration of 5.times.10.sup.18 cm.sup.-3 and 100 .mu.m thick, having an energy band gap corresponding to light of wavelength of .lambda.g=0.9 .mu.m.
Next, a first photoresist mask having a width of 1.3 .mu.m and extending in the longitudinal direction is produced on the wafer at the center of the element, and the undoped InGaAsP layer 2 is etched with H.sub.2 SO.sub.4, thereby producing a light absorption layer 2 of 1.3 .mu.m width, 0.13 .mu.m height, and 300 .mu.m length. After removing the first photoresist mask, a p type InP layer 3, having a dopant concentration of 1.times.10.sup.18 cm.sup.-3 and 2.13 .mu.thick, having an energy band gap corresponding to light of wavelength .lambda.g=0.9 .mu.m is epitaxially grown thereon.
Then, Ti of 500 .ANG. thickness and Au of 2500 .ANG. thickness are electron beam deposited on the p type InP layer 3 thereby to produce a Ti/Au electrode which functions as a p side electrode 4 for inputting a modulation signal 7 to the light absorption layer 2. On the other hand, AuGe of 800 .ANG. thick and Au of 2500 .ANG. thick are electron beam deposited on the n type InP substrate 1, thereby producing an AuGe/Au electrode which functions as an n side electrode 5.
A description is given of the operation.
FIG. 10 is a diagram illustrating the absorption spectrum obtained when an electric field is applied to between the p side electrode 4 and the n side electrode 5 in the semiconductor light intensity modulator of FIG. 9. In the figure, reference numeral (3) represents a relation between the wavelength .lambda. and the light absorption quantity (a) when no electric field is applied between the p side electrode 4 and the n side electrode 5, reference numeral (4) represents a relation between the wavelength .lambda. and the light absorption quantity (a) when a negative electric field is applied to the p side electrode 4 relative to the n side electrode 5. When light of wavelength 1.55 .mu.m is incident, the light absorption amount (a) of the curve (3) when no electric field is applied to the p side electrode 4 is 0, while the width of the absorbing region of the spectrum of the curve (4) when a negative electric field is applied to the p side electrode 4 relative to the n side electrode 5 is broadened toward a longer wavelength, thereby meaning that light of wavelength 1.55 .mu. m is absorbed by an amount of .DELTA.a.
For example, when light of wavelength 1.55 .mu.m is incident on the facet of the modulator in a state when no voltage is applied to the light absorption layer 2, the light absorption layer 2 absorbs no light, and thereby the light is output from the opposite side facet through the light absorption layer 2 without being absorbed. Meanwhile, when a reverse bias voltage of -2 V is applied between the p type InP layer 3 and the n type InP substrate 1, thereby applying an electric field to the light absorption layer 2, the light absorption layer 2 also absorbs light of wavelength 1.55 .mu.m which is longer than the wavelength .lambda.g=1.4 .mu.m corresponding to the energy band gap of the light absorption layer 2, due to the electric field absorption effect.
The electric field absorption effect will be hereinafter described. This electric field absorption effect is called the "Franz-Keldysh effect". The Franz-Keldysh effect is a result of the dependence of the fundamental absorption spectrum of semiconductor or insulator on electric field, and this phenomenon was independently predicted by W. Franz and L. V. Keldysh in 1958. This Franz-Keldysh effect is due to the existence of an electric field gradient so that electrons occupying the valance band absorb light as well as transit to the conduction band due to the tunneling effect, thereby enabling absorption of light of an energy smaller than the energy band gap of the material. When an electric field is applied, the absorption the light tails become lower, toward the low energy side of the absorption edge. In addition, vibrating components appear in the absorption spectrum due to an electric field at the high energy side of the absorption edge (this is called a vibration type Franz-Keldysh effect), and these phenomena are actually observed at an electric field of about 10.sup.4 V/cm for a large number of semiconductors.
As shown in FIG. 10 and as already described, this light absorption layer 2 absorbs no light of a longer wavelength than .lambda.g=1.4 .mu.m corresponding to the energy band gap of that layer when no electric field is applied, and it even absorbs light of wavelength 1.55 .mu.m, which is longer than .lambda.g=1.4 .mu.m, when an electric field is applied. Further, as shown in FIG. 10,
(3) when E=0, the light absorption of light of the wavelength 1.4 .mu.m is 4000/cm and there is no absorption of the light of wavelength 1.55 .mu.m, and
(4) when E&lt;0, the light absorption amount of the wavelength 1.4 .mu.m is smaller than 3000/cm, but the light absorption amount .DELTA.a of the wavelength 1.55 .mu.m is 1000/cm, where the total light absorption amount when E=0 and the total light absorption amount when E&lt;0 are equal to each other.
As described above, the prior art semiconductor light intensity modulator utilizes the Franz-Keldysh effect, and it operates so that when a modulation signal 7 is input to the semiconductor light intensity modulator, light of a particular wavelength among the light passing through the light absorption layer 2 is absorbed depending on the reverse bias voltage applied thereto, whereby the light amount transmitted through the semiconductor light intensity modulator is varied.
The prior art semiconductor light intensity modulator is constructed as described above, and when an electric field is applied to the semiconductor, the absorption of light varies and the refractive index of the semiconductor varies, thereby changing the phase of the transmitting light, reducing the monochromaticity of the light. Accordingly, when this modulator is utilized in an optical communication system, the distance over which the transmission is possible is shortened because the phase modulation due to the variation in the refractive index accompanying the change in the light absorption.